1. Field of the Invention The invention relates to interface circuits for use in telephone systems and more particularly to a trunk interface circuit including a current compensated transformer.
2. Discussion of Related Art
In a telephone network, trunks are used to provide links between various telephone switching facilities. In the case of a two-wire trunk used for example between a private branch exchange (PBX) and a central office (CO), a PBX trunk interface circuit and a CO trunk interface circuit are used to provide termination for the trunk. Each of the trunk interface circuits are required to couple a.c. signals between the trunk and the respective associated telephone facility. In addition the trunk interface circuits are required to apply and respond to supevisory signalling which includes application of C.O. office battery across the two wire trunk in forward and in reverse directions. A transformer in the PBX trunk interface circuit includes a core of magnetic material which carries at least two windings. One of the windings is for connection in series between the leads of the trunk and the other is connected to a communication port of the PBX. As supervisory signalling direct current is conducted via the series connected winding, the transformer core is provided in sufficient bulk to prevent saturation of the core from d.c. flux associated with the supervisory signalling direct current.
Examples of miniaturized current compensated transformers in line circuits are disclosed by Max S. Macrander in U.S. Pat. No. 3,714,548 issued Jan. 1973; John F. O'Neill in U.S. Pat. No. 4,046,967 issued Sept. 1977; and Vincent V. Korsky in U.S. Pat. No. 4,103,112 issued July 1978. In each of these examples, subscriber loop energizing direct current is fed in one direction to a loop circuit through a transformer winding, while a complementary poled winding is caused to conduct a unidirectional balancing or compensating current. In contrast a trunk interface circuit is required to pass direct current in two directions, i.e. forward and reverse. In the case of a PBX trunk interface circuit a.c. signal coupling is required in the presence of remotely supplied supervisory signalling current flowing in either of two directions. Hence these trunk interface circuits have continued to be manufactured without the benefits of transformer miniaturization.